The use of mobile communications networks has increased over the last decade. Operators of the mobile communications networks have increased the number of base stations in order to meet an increased demand for service by users of the mobile communications networks. Radio signals are typically relayed into a cell of the mobile communications network, and vice versa.
It is of interest to provide a reliable quality of service to an individual user of the mobile communications network given the increase in the number of users. Several techniques have been suggested in order to deal with the increased number of users within the mobile communications network. None of the several techniques comprises beamforming capabilities in order to direct a beam relayed by the base station in different directions to improve service coverage within the cells of the mobile communications network. Beamforming may be achieved with an array of antenna elements. The beamforming techniques rely on defined phase and amplitude relations between individual ones of the antenna elements of the antenna array. A transmit path and/or a receive path is associated with at least one antenna element. Calibration of the transmit paths and/or the receive paths is required to provide the defined phase, amplitude and delay relationship between the individual ones of the antenna elements. The calibration allows the estimation of phase, amplitude and delay deviation accumulated along individual transmit paths of the antenna array. Likewise, the calibration comprises estimating phase, amplitude and delay deviations accumulated along individual ones of the receive paths. In a second step, the phase, amplitude and delay deviation accumulated along the transmit paths can be corrected. An appropriate phase and amplitude change may be imposed or applied to the individual transmit/receive paths to yield the defined phase and amplitude relationship between the individual transmit/receive paths of the antenna array, in order to allow for beamforming techniques.
The transmit paths and/or the receive paths typically differ slightly in their behavior towards amplitude, phase, and delay. These differences may be caused, for example, by different signal path lengths from one transmit path and/or receive path to another. For meaningful beamforming to be possible, these differences must be taken into account, i.e., the antenna array needs to be calibrated so that each antenna element relays an assigned portion of the radio signal in a desired and expected manner. The term “relaying” applies to the uplink direction or to the downlink direction or to both.
In the past, the calibration of an antenna array often used a calibration signal or “sounding” signal. FIG. 1 shows a basic outline of a receive-path calibration system based on a calibration signal for use in an active antenna array. The active antenna array comprises a plurality of transceive paths 1 to n. In FIG. 1, only three of the transceive paths are illustrated, for the sake of clarity. Furthermore, only the receive paths are shown in FIG. 1, but not the transmit paths. Taking the transceive path 1 as an example, transceive path 1 is connected to an antenna element (not shown) at the left. A receive signal picked up by the antenna element passes a signal coupler substantially unchanged. The receive signal then reaches a duplex filter which acts to inject the receive signal into an actual receive path. The duplex filter functions on the basis of frequency filtering and separates a frequency spectrum reserved for the receive band from a frequency spectrum reserved for the transmit band (from the perspective of a base station). In FIG. 1, the receive path comprises a further signal coupler, which will be explained later. The receive path also comprises a low noise amplifier (LNA), a bandpass filter, and a delta-sigma analogue-to-digital converter. These three components of the receive path provide for amplification, filtering, frequency conversion, and digitalization of the receive signal to provide a digitized receive signal. The digitized receive signal is supplied to a digital signal processor (DSP) for further processing, such as descrambling and distribution to a plurality of user channels.
The digital signal processor is also used for generating a calibration signal. In the arrangement shown in FIG. 1, the DSP generates a calibration signal at baseband or at an intermediate frequency (IF), which is then frequency converted by means of a delta-sigma digital-to-analogue converter to a radio frequency (RF). A bandpass filter at the output of the delta-sigma digital-to-analogue converter filters the up-converted calibration signal and in particular removes any undesired quantization noise produced by the delta-sigma digital-to-analogue converter. The up-converted calibration signal is now provided to one of the signal couplers to the left and the right of the duplex filter. Clearly, the left signal coupler shown in dashed line illustrates an alternative embodiment. Whilst it is clearly possible to inject the required calibration signal at any point in the receive paths, the two most likely locations are shown in FIG. 1. Note that both locations would normally not be used simultaneously, although it is possible that the calibration signal could be switched from one to the other, e.g. for a factory calibration of the whole system, using the left-most coupler, versus an operational calibration of the low noise amplifiers, analogue-to-digital converters, etc., using the right-hand of the two couplers.
The signal couplers are designed for a major portion of the calibration signal to be injected in the receive direction, i.e., to the right in FIG. 1. Ideally, only a very small fraction of the calibration signal is injected in the transmit direction, i.e., from right to left. Injecting the calibration signal in the transmit direction could lead to undesired reflections at the antenna element and/or the duplex filter, or it could be radiated by means of the antenna element to the environment. The portion of the calibration signal that has been injected into the receive path in the receive direction undergoes the same signal processing as the receive signal. Once the calibration signal has made its way back to the digital signal processor, the digital signal processor extracts the calibration signal from the rest of the receive signal. By comparing the originally generated calibration signal with the calibration signal as received from the receive path 1, it can be determined how the receive path modifies the amplitude, phase, and delay of the receive signal.
In an active antenna array used for beamforming, one is mostly interested in relative differences between the transmit paths and/or receive paths. Therefore, it is not necessary to know how the delta-sigma digital-to-analogue converter and the bandpass filter in the calibration signal path modify the calibration signal in absolute terms. Indeed, any modification of the calibration signal in the calibration signal path will practically be the same for all of the transceive paths (with the exception of the mentioned relative differences) and therefore will not have a significant influence on the determination of the relative differences between the transceive paths.
The known calibration scheme works well, but requires a significant amount of additional hardware, in particular a signal coupler in each receive path. Moreover, from the perspective of the receive signals, the calibration signal is noise and therefore reduces the signal-to-noise ratio (SNR) of the receive paths, at least while calibration of a particular receive path is taking place.
US Patent Application Publication 2001/0009861 A1, titled “Bootstrapped, Piecewise-Asymptotic Directivity Pattern Control Mechanism Setting Weighting Coefficients of Phased Array Antenna” and assigned to Harris Corporation, describes an alternative method. In US 2001/0009861, weighting coefficients for a phased array antenna are iteratively refined to optimal values by a “bootstrapped” process that starts with a coarse set of weighting coefficients, to which received signals are subjected, to produce a first set of signal estimates. These estimates and the received signals are iteratively processed a prescribed number of times to refine the weighting coefficients, such that the gain and/or nulls of antenna's directivity pattern will maximize the signal-to-noise ratio. The method comprises, amongst others, the generation of signal transforms, the generation of a noise-signal matrix, the generation of covariance matrices, and the calculation of a matrix product. Accordingly, data processing needed for performing the method proposed in US 2001/0009861 A1 is rather involved. Furthermore, the method provides “lumped” weighting coefficients that are valid for a particular user. The method does not appear to provide for a determination of a constant, systematic deviation between the data processing of the various receive paths.
U.S. Pat. No. 6,031,877, assigned to Motorola, Inc., discloses an apparatus and method for adaptive beamforming in an antenna array. A predictive filter supplies an estimate of receive signal samples likely to be received in a burst immediately preceding a transmission. Combination of this estimate with received signal samples obtained from actual (historically received) signals, received over a predetermined number of frames, yield estimates of optimum beamforming coefficients for application to data for transmission from an adaptive array of antenna elements. The method comprises, amongst others, the calculation of a received signal cross-correlation matrix with respect to a received signal vector at n branches (i.e. n antenna elements). As was the case with US 2001/0009861 A1, U.S. Pat. No. 6,031,877 does not appear to disclose that e.g. the phase differences between two receive paths comprise constant, systematic differences which are due to the imperfections of manufacture of the various receive paths or environmental influences such as different temperatures.